The synaptic organization of the dorsal raphe nucleus of the rat will be studied using intracellular recording techniques, intracellular staining of recorded neurons with horseradish peroxidase (HRP), and anterograde labeling of structures projecting to dorsal raphe with the tracer phaseolus vulgaris leucoagglutinin (PHAL). The morphological studies are chiefly light microscopic but are supplemented by verification of key observations at the ultrastructural level. The initial anatomical studies will use PHAL to label those cell groups in brainstem and midbrain that have been shown to send axons to the dorsal raphe. The strength and topography of the projections will be determined in this way and the course of the afferent axons mapped. Without the latter information, no sense could be made of the recording experiments that follow. Intracellular recordings will be made in order to determine the nature of the post-synaptic response evoked by stimulation of midbrain and brainstem afferents to dorsal raphe. Experimental approaches designed to isolate conflicting synaptic inputs and to verify that they involve monosynaptic pathways will be used. HRP will be routinely injected into the recorded neurons in order to provide a combined morphological and physiological basis for identifying the postsynaptic neuron from among the subtypes that exist in dorsal raphe. Intracellular labeling with HRP also leads to the final aspect of this project which is to conduct neuroanatomical tracing experiments at the neuronal level of resolution. In double labeling experiments, the distribution of PHAL labeled axon terminals in close apposition to HRP filled neurons, as seen with The light microscope, will be plotted in three dimensional reconstructions. the resulting maps are of what I define to be putative synaptic contacts. While they are a useful end product in themselves, the assertion that close apposition do represent synaptic contacts will be tested by electron microscopy, both of a sample of close apposition in the standard double labeled material and in single, PHAL, labeled cases.